Some electronic devices, such as cellular phones and smart phones, have menus or graphical user interfaces that are rather complex. In order to navigate through these menus, a user may move his finger over a navigator, which causes an icon associated with the menus to move. As electronic devices get smaller, the navigators must also get smaller so as not to limit the size of the electronic devices.
An Optical Finger Navigation (OFN) system is often incorporated into such electronic devices. An OFN system traditionally includes an illumination system which lights up a target area and an optical imaging system which receives images of a user's finger as it interfaces with the target area. The images received at the optical imaging system can then be converted into electrical signals which are used to manipulate objects displayed by the electronic device.
The illumination system has several parameters that can be altered to achieve superior finger tracking performance. The first parameter is the area of the beam spot which illuminates the target area—the larger the beam spot the more image data that is potentially available to the sensor in the imaging system. The second parameter is the amount of angular spread of light rays incident on the object that is proximate to the target area—a more accurate tracking performance can be achieved when the amount of angular spread is minimized. The third parameter is the amount of optical power (irradiance) that falls onto the target area—better images and more accurate tracking can be achieved if there is sufficient irradiance falling onto the target area.
One existing solution for manipulating and maximizing these illumination system parameters to achieve quality tracking performance utilizes light pipes that collect, expand, and direct (collimate) light from a bare LED die onto the target area.
Another existing solution for manipulating and maximizing these illumination system parameters to achieve quality tracking performance utilizes a packaged LED (usually with an integrated collection of optics such as reflector cups and domes) and then redirects light emitted by the packaged LED onto the target area via mirrors, light pipes, and the like.
The main drawback to both of the above-described existing solutions is that they require valuable space to achieve quality tracking performance. In particular, both of these solutions typically require a vertical space (thickness) in the region of about 2 mm to about 4 mm. As there is a need for smaller electronic devices and smaller OFN systems, there is also a need to rethink the existing illumination systems used in OFN systems.